Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver

[Will Deacon]

On Wed, Jul 19, 2017 at 05:09:05PM +0530, Anup Patel wrote:
> On Wed, Jul 19, 2017 at 5:03 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Wed, Jul 19, 2017 at 05:01:11PM +0530, Anup Patel wrote:
> >> On Wed, Jul 19, 2017 at 4:55 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> >> > On Wed, Jul 19, 2017 at 04:53:04PM +0530, Anup Patel wrote:
> >> >> On Wed, Jul 19, 2017 at 4:30 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >> >> > On 19/07/17 10:33, Anup Patel wrote:
> >> >> >> The ARM SMMUv3 support bypassing transactions for which domain
> >> >> >> is not configured. The patch adds corresponding IOMMU capability
> >> >> >> to advertise this fact.
> >> >> >>
> >> >> >> Signed-off-by: Anup Patel <anup.patel@xxxxxxxxxxxx>
> >> >> >> ---
> >> >> >>  drivers/iommu/arm-smmu-v3.c | 2 ++
> >> >> >>  1 file changed, 2 insertions(+)
> >> >> >>
> >> >> >> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> >> >> >> index 568c400..a6c7f66 100644
> >> >> >> --- a/drivers/iommu/arm-smmu-v3.c
> >> >> >> +++ b/drivers/iommu/arm-smmu-v3.c
> >> >> >> @@ -1423,6 +1423,8 @@ static bool arm_smmu_capable(enum iommu_cap cap)
> >> >> >>               return true;
> >> >> >>       case IOMMU_CAP_NOEXEC:
> >> >> >>               return true;
> >> >> >> +     case IOMMU_CAP_BYPASS:
> >> >> >> +             return true;
> >> >> >
> >> >> > And this is never true. If Linux knows a device masters through the
> >> >> > SMMU, it will always have a default domain of some sort (either identity
> >> >> > or DMA ops). If Linux doesn't know, then it won't have been able to
> >> >> > initialise the stream table for the relevant stream IDs, thus any
> >> >> > 'bypass' DMA is going to raise C_BAD_STE. SMMUv3 can effectively only
> >> >> > bypass unknown stream IDs if disabled entirely.
> >> >>
> >> >> What if we don't want to use IOMMU for certain device and
> >> >> due to this we never provide "iommus" DT attribute in the
> >> >> device DT node. Further, we want to access device without
> >> >> "iommus" DT attribute from user-space using VFIO no-IOMMU.
> >> >
> >> > Wait, you want to pass a device through to userspace but you don't want to
> >> > use the IOMMU? Why not?
> >> >
> >> > If you describe the SMMU in firmware with only a partial topology
> >> > description, then you will run into problems with unknown masters trying to
> >> > perform DMA. That's the IOMMU doing its job!
> >>
> >> We are keeping disable_bypass = false. In other words, we
> >> are using bypass mode for unmatched streams. The real
> >> reason is limited number of SMRs due to which we choose
> >> not to provide "iommus" DT attribute for certain devices.
> >
> > Understood, but that's not robust for SMMUv3 and we *really* shouldn't have
> > a user ABI that changes behaviour based on a cmdline option. VFIO should be
> > requesting its own identity mappings, if that's what you need.
> 
> Currently, the iommu_present() check in vfio_iommu_group_get()
> is preventing us allow no-IOMMU mode for certain devices.
> 
> Is there a better replacement of iommu_present() check in
> vfio_iommu_group_get()?

There are two things here:

  1. iommu_present() is pretty useless, because it applies to a "bus" which
     doesn't actually tell you what you need to know for things like the
     platform_bus, where some masters might be upstream of an SMMU and
     others might not be.

  2. If a master *is* upstream of an IOMMU and you want to use no-IOMMU,
     then the VFIO no-IOMMU code needs to be extended so that it creates
     an IDENTITY domain on that IOMMU.

I think you need to solve both of those to make this robust. That way,
VFIO can accurately tell whether a given master has an IOMMU or not and
then figure out whether or not it needs to create an IDENTITY domain,
or whether it can do like it does at the moment and ignore the IOMMU
entirely (for the no-IOMMU case).

Will